The present invention relates to process field devices of the type used to monitor or control operation of an industrial process. More specifically, the present invention relates to process field devices which communicate on a two wire process control loop and which are completely powered with power received from the two wire process control loop.
Process devices are used in industrial process control systems to monitor and/or control industrial processes. A control device is a process device which is used to control the process. Example control devices include pumps, valves, actuators, solenoids, motors, mixers, agitators, breakers, crushers, rollers, mills, ball millers, kneaders, filters, blenders, cyclones, centrifuges, towers, dryers, conveyors, separators, elevators, hoists, heaters, coolers, and other such equipment. A transmitter is a process device which is used to sense (or monitor) operation of the process, for example by monitoring a process variable such as temperature, pressure, flow, etc. The monitored process variable is transmitted so that it can be used by other equipment in the process, for example by a central control room. Another example process device is a process monitor or communicator which is used to monitor operation of the process, equipment used in the process such as process transmitters or process controllers, and control process devices, for example by programming or sending instructions to the device.
Two wire process field devices are process devices which are located at a remote location or otherwise require electrical power which is received entirely through a connection to a two wire process control loop. A two wire process control loop refers to a wiring system utilizing two wires and which is used to carry information related to the process being monitored or controlled. For example, one standard type of two wire process control loop uses a 4–20 mA current level to represent a process variable. In such a configuration, a transmitter can control the current level to a value, such as 10 mA, which is representative of a sensed process variable such as pressure. Other process control loop configurations allow for the transmission of digital signals. The digital signals can be an alternative to transmission of an analog current level or can be in combination with transmission of the analog current level.
In order to reduce the amount of wiring required to couple to a process device, many process devices are entirely powered with power received from the two wire process control loop. Thus, the total amount of power available to the process device is limited. For example in a 4–20 mA current loop, the total power available is limited by the lowest level at which the current level can be set (for example 3.6 mA) and the maximum voltage drop available across the device (for example something less than 24 volts) in an intrinsic safe location. This is less than 0.9 watts available to power the device.
In many cases, the functionality of a process device is limited by the amount of power which is available from the two wire process control loop. For example, additional functionality may require addition computational capabilities from a microprocessor. This increased computational power requires greater electrical power and may exceed the power budget available from the loop. In order to increase the amount of power available to circuitry in a process field device, field devices typically utilize highly efficient power supplies to convert power received from the two wire process control loop into a regulated voltage level for use by internal circuitry. One type of power supply, the switching regulator has been used in process devices due to its efficiency in providing a regulated supply voltage to components in a field device. For example, the use of switching regulators is described in U.S. Pat. No. 5,535,243, which issued Jul. 9, 1996, entitled POWER SUPPLY FOR FIELD MOUNTED TRANSMITTER and U.S. Pat. No. 5,973,942, which issued Oct. 26, 1999, entitled START UP CIRCUIT FOR DC POWERED FIELD INSTRUMENT. However, a switching regulator may introduce noise into the process control loop which may alter or otherwise detrimentally affect transmission of information on the loop.